Market adoption of wireless LAN (WLAN) technology has exploded, as users from a wide range of backgrounds and vertical industries have brought this technology into their homes, offices, and increasingly into the public air space. This inflection point has highlighted not only the limitations of earlier-generation systems, but also the changing role that WLAN technology now plays in people's work and lifestyles across the globe. Indeed, WLANs are rapidly changing from convenience networks to business-critical networks. Increasingly users are depending on WLANs to improve the timeliness and productivity of their communications and applications, and in doing so, require greater visibility, security, management, and performance from their network.
Multicast is the delivery of information to a group of destination nodes simultaneously over a network. In some networks, a multicast message is one that is transmitted to selected multiple recipients who have joined a corresponding multicast group. The sender has to generate only a single data stream. A multicast-enabled router or other network element generally forwards a multicast message to a particular network only if there are multicast receivers on that network.
Clients typically subscribe to multicast streams using a subscription protocol. In many network deployments, the delivery of multicast traffic involves the dynamic configuration of one or more hierarchical routing and/or switching topologies (multicast trees) among nodes (such as routers, distribution switches, central controllers, access points, etc.). In some implementations, clients transmit join requests that are snooped by one or more network elements in the network infrastructure that process the message and possibly join the hierarchical multicast tree for that stream. In some deployments, the source of the multicast stream is the root of the multicast tree. At any given time, there may be multiple separate multicast trees in a network given the disparate possible sources of multicast traffic.
Beacon frames are transmitted by an access point at regular intervals to announce the existence of the wireless network, and are also used for synchronization purposes. The default behavior is to send a beacon frame once every 100 milliseconds. Beacon frames typically include a Traffic Indication Map information element (TIM IE). In some beacon frames, the TIM IE includes a Delivery Traffic Indication Map (DTIM) message. These beacons are referred to as DTIM beacons. These special DTIM beacons are sent at an interval specified in the DTIM period (measured in beacon intervals), also contained in every beacon. The DTIM beacon alerts the clients that multicast and broadcast packets buffered at the AP will be transmitted immediately after the transmission of this beacon frame. The larger the DTIM period, the longer the delay between the deliveries of multicast frames. After each DTIM interval, the associated wireless clients, in a power save mode, wake up from a power-conservation state in order to receive the multicast stream and then return to the power-conservation state.